To improve quality of textiles, it is required to detect hetero fibre and different heterogeneous colors in homogeneous fibre, which is collectively referred to as yarn clearing. In the world today, widely-used yarn clearing methods include a photodetection method, a capacitive detection method, and a detection method combining both.
Principles of the photodetection method are as shown in FIG. 1, including a light emitting diode 1, a yarn 2, and a detector 3 sequentially arranged. The light emitting diode directly irradiates on the to-be-detected yarn, the yarn blocks a part of light emitted from the light emitting diode, and the detector inspects the quality of the yarn by detecting variation of luminous flux blocked. In this method, the variation of the luminous flux blocked by the yarn is used as an assessment variable. Hence, generally only those quality problems having a negative effect on the sectional area of the yarn can be detected, for example, slubs and neps of the yarn, short and thick, long and thick, long and thin. But this method is unable to detect hetero fibre or homogeneous heterochromous fibre.
Some foreign electronic yarn clearers adopt the method combining the photodetection and the capacitive detection. This method can detect hetero fibre and homochromatic fibre, but is complex in design and high in cost.